The present invention relates generally to a method and apparatus for transmitting speech signals in a digital radio communication system, and is specifically concerned with a speech signal transmitting method and apparatus which utilizes what is called VOX (Voice Operated Transmitter) control in which in transmitting speech signals, only for a speech-active duration (a time period when speech actually exists) the transmission of the speech signal is carried out, in other words, the power for the transmitter is turned on, and for a silent duration (a time period when any speech does not exist) the power for the transmitter is turned off thereby reducing the amount of power consumed by the transmission of speech signals.
In radio equipments such as portable telephones, cordless telephones and the like, there is an increased demand for more lightweight and more compact equipment and an effective approach to satisfying that demand would be to reduce the amount of power consumed by a radio equipment thereby decreasing the capacity requirements of the battery used therein. To this end, there has been proposed a method which employs a VOX (Voice Operated Transmitter). This method is one that stops the transmission of radio waves while the talker is not actually speaking (i.e. for a silent duration) and transmits radio waves only while he is actually speaking (i.e. for a speech-active duration). For example, assuming that the speech activity factor (=speech-active duration/(speech-active duration+silent duration)) is 50%, the use of the VOX affords reduction of power consumption substantially by half that of ordinary radio equipment.
For the operation of the VOX, it is necessary to allow the receiving side to make a distinction between the speech-active duration and the silent duration. To meet this requirement, a method has been proposed in which a speech signal including at least one speech active duration (in which an actual speech exists) and one silent duration (speech pause duration in which no speech exists) is transmitted from the transmitting side by inserting, at the starting portion of a speech-active duration, a speech-active duration start information (hereinafter referred to as a preamble) which indicates the beginning of the speech-active duration in place of a coded information of the actual speech at that portion of the speech-active duration, and inserting at the end of the speech-active duration a speech-active duration end information (hereinafter referred to as a postamble) which indicates the termination of the speech-active duration, in other words, the beginning of the silent duration, respectively. The present invention utilizes this conventional method.
The application of this method to an automobile telephone system involves some problems to be solved, and in particular, substantially no study has been given the VOX system for use with a digital mobile radio communication system.
The present invention is intended to solve three problems experienced in the prior art. In the first place, since mobile communications are inevitably accompanied by channel errors, there is the possibility of the preamble being not correctly transmitted to the receiving station. In such an instance, even if a speech-active duration subsequent to the preamble can be received normally, no decoding operation can be initiated.
Another problem is as follows: With a view to making effective use of radio waves, digital mobile communications utilize various high efficiency speech coding schemes such as CELP (Code Excited Linear Predictive coding), VSELP (Vector Sum Excited Linear Predictive coding), TC-WVQ (Transform Coding with Weighted Vector Quantization), etc. In these schemes the speech is partitioned into short time frames of about 20 mS duration, for instance, and is coded for each frame, and it is conventional to use a method called "prediction" which utilizes a previous speech sequence to reproduce or regenerate the subsequent speech in order to allow high efficiency compression of the speech. In the predictive coding method the decoding of coded speech data of each frame calls for the result of decoding of the immediately preceding speech frame. Hence, it is unavoidable in the prior art that a speech starting frame corresponding to the beginning of the speech-active duration, which is transmitted directly after the preamble which is not a speech frame, cannot correctly be received at the receiving station, allowing an abnormal sound to be produced. Moreover, mobile communication terminals are often used outdoors in an environment with background noise. It is unnatural, however, that such background noise cannot be heard at the receiving station during the suspension of transmission which is one of VOX functions. No measures have been taken against these problems.